Reflective color results from one of two primary mechanisms. One mechanism is the traditional subtractive mechanism where a chromophore molecule absorbs a particular wavelength of light and reflects the remaining light of a given color. This is the mechanism of pigments.
Addressable reflective chromophore type color can be seen in several electrophoretic display technologies, such as certain E-Ink and Liquavista products and a number of products that fall under the definition of transflective displays. Generally speaking, electrophoretic approaches cannot refresh at a rate sufficient for animated display and exhibit poor color saturation, while the transflective displays, while having high pixel refresh rates, yield a poor color image quality and are significantly more expensive than a standard liquid crystal display (LCD).
Another group of color mechanisms fall under the general term of “structural” color. Bragg reflectors, diffraction gratings, and other similar mechanisms exist in this classification. Such devices reflect color based upon the periodicity of a repeated structure. Structural color, in nature, can exhibit a broad color space and is often of striking color saturation.
One particular structural mechanism that was initially described by Lord Rayleigh in 1917 (see S. Kinoshita, “Structural Colors in the Realm of Nature”, 2008) is that of the photonic crystal. Arsenault (Nature Photonics, 1, pg 468, 2007) describes an approach that builds a Bragg reflector using three dimensional photonic crystals wherein the crystal and, consequently, the Bragg refraction grating giving color, can be changed by changing the three dimensional geometry of the spherical photonic crystal elements. Photonic crystals are materials with a periodic modulation in refractive index. The characteristics of the crystals or
Bragg reflector can be tuned to only reflect a narrow range of wavelengths (colors) due to constructive and destructive interference of the impinging light waves.
Structural color displays are rare (if extant at all) and no existing commercial device is addressable with either a slow or fast pixel transition.
Backlit color LCDs are very inefficient since, typically, less than 10% of the back light flux passes through the liquid crystal layer, even with all light valves fully opened. Existing reflective color displays provide poor color and refresh rates, although they may be energy efficient and use ambient white light.
What is needed is an improved reflective color display that is energy efficient, provides good color saturation, is capable of good resolution, enables a high refresh rate for displaying moving images, can be made thin, and can be manufactured inexpensively.